L-Arginine-Modified Silica for Adsorption of Gold(III) | Hastuti | Indonesian Journal of Chemistry 21203 40288 1 PB
108
Indones. J. Chem., 2015, 15 (2), 108 - 115
L-ARGININE-MODIFIED SILICA FOR ADSORPTION OF GOLD(III)
Sri Hastuti1,*, Nuryono2, and Agus Kuncaka2
1
Department of Chemistry, Faculty of Mathematics and Natural Sciences, Sebelas Maret University,
Jl. Ir. Sutami 36 A Kentingan, Surakarta 57126, Indonesia
2
Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada,
Sekip Utara, Yogyakarta 55281, Indonesia
Received December 1, 2014; Accepted March 17, 2015
ABSTRACT
In this research, L-arginine-modified silica (SiO2-Arg) with 3-glycidoxypropyl-trimethoxysilane (GPTMS) as the
linking agent has been synthesized through sol gel process for adsorption of Au(III) in aqueous solution. Tetraethyl
orthosilicate (TEOS) as the silica source precursor, L-arginine solution 0.9 M with various volume ratios and the
linking agent were mixed together to form a gel. SiO2-Arg was characterized using Fourier transform infrared (FTIR)
spectrophotometer, thermogravimetric analysis (TGA), and an elemental analysis. Adsorption was carried out in a
batch system under various experimental conditions including contact time and initial concentration of metal Au(III).
The selectivity of adsorbent toward Au(III) was examined in the presence of Cu(II), Fe(III), and Zn(II) ion at various
pHs. Results of characterization showed that silica has been successfully modified with L-arginine. The optimum
adsorption of Au(III) on SiO2-Arg was obtained at pH of 3.0 and the adsorption isotherm of Au(III) on SiO2-Arg gives
the adsorption capacity of 52.79 mg/g. The kinetic study demonstrates that the adsorption of Au(III) ion follows
–1
–1
pseudo-second order with the rate constant of 53197 g mol min . The selectivity order of Au-Zn = Au-Cu > Au-Fe.
This sol-gel preparation is simple and provides prospective application of SiO2-Arg material as an effective
adsorbent for metal ions particularly gold(III).
Keywords: silica; modification; L-arginine; adsorption; gold(III)
ABSTRAK
Dalam penelitian ini, silika termodifikasi L-arginin (SiO2-Arg) dengan 3-glisidoksipropil-trimetoksisilan (GPTMS)
sebagai penggandeng telah disintesis melalui proses sol gel untuk adsorpsi Au(III) dalam larutan air. Tetraetil
ortosilikat (TEOS) sebagai prekursor sumber silika, larutan L-arginin 0,9 M dengan berbagai perbandingan volume
dan zat penggandeng dicampur bersama-sama untuk membentuk gel. SiO2-Arg dikarakterisasi menggunakan
spektropotometer Infra Merah, analisis termogravimetri, dan analisis unsur. Adsorpsi dilakukan dalam sistem batch
pada berbagai kondisi termasuk waktu kontak dan konsentrasi awal ion logam Au(III). Selektivitas adsorben
terhadap Au(III) diuji dengan hadirnya ion logam Cu(II), Fe(III), dan Zn(II) pada berbagai pH. Hasil karakterisasi
menunjukkan bahwa silika telah berhasil dimodifikasi dengan L-arginin. Adsorpsi optimum Au(III) pada SiO2-Arg
diperoleh pada pH 3,0 dan isoterm adsorpsi Au(III) pada SiO2-Arg memberikan kapasitas adsorpsi 52,79 mg/g. Studi
–1
kinetika menunjukkan bahwa adsorpsi ion Au(III) mengikuti pseudo orde dua dengan konstanta laju 53197 g mol
–1
min . Urutan selektivitas Au-Zn = Au-Cu > Au-Fe. Preparasi sol-gel ini sederhana dan memberikan aplikasi yang
prospektif dari material SiO2-Arg sebagai adsorben yang efektif terutama untuk ion logam emas(III).
Kata Kunci: silica; modifikasi; L-arginin; adsorpsi; emas(III)
INTRODUCTION
Recovery of gold from waste water is more
interesting than that of most other metals [1]. Moreover
gold recovery from aqueous solutions has received
significant attention because gold is present in
appreciable amounts in electronic parts and plating
materials [2]. The content of gold in electronic waste is
greater than the gold content of the ore. For example,
gold concentrations in mobile phone handsets and
* Corresponding author. Tel/Fax : +62-85725590941
Email address : hastuti.uns@gmail.com
Sri Hastuti et al.
computer circuit boards are about 300-350 and
200-250 g/t, respectively [3] while the ore contains gold
in a range of 5-30 g/t [4].
There are several methods for the removal of
gold(III) from aqueous solutions, such as solvent
extraction [5-6] and adsorption [1]. The last method
seems to be the most suitable for the recovery of
gold(III) in the case of low concentration due to low
cost and high efficiency. In development of adsorbent,
silica gel is very important material because it can be
Indones. J. Chem., 2015, 15 (2), 108 - 115
used as a support material possesses some advantages
such as high mechanical and chemical properties, high
porosity, large surface, no swelling, resistant in microbial
attack, resistant in decay radiation and heat-stable
[7-10]. In addition, chelating agents can be easily
bonded chemically to the support, that affording a higher
stability [7]. Chemically, silica contains functional groups
of silanol (-Si-OH) and siloxane (-Si-O-Si-) on the
surface, allowing to be modified with active groups
specific to certain targets [11-13].
One way to improve the ability of silica as an
adsorbent is by surface modification with organic
functional groups. Modification may be chemically
conducted by using organosilane compound as a linker,
which then is followed by attachment of the active group
to the linker. The various linker has been used for the
modification of silica, such as 3-aminopropyltriethoxy
silane (APTES) [14], 3-chloropropyltrimethoxysilane
(CPTMS) [15], 3-aminopropyltrimethoxysilane (APTMS)
[16-17], 3-glycidoxypropyltrimethoxysilane (GPTMS)
[18]. Donor atoms commonly used as the active group
include nitrogen (e.g. amine, azo, amide and nitrile),
oxygen (e.g carboxyl, hydroxyl, carbonyl and
phosphoryl), and sulfur (e.g. thiocarbamate, thioethers,
and mercapto) [8]. The selectivity of the immobilized
surface towards Au(III) depends on various factors such
as kind of complexing agent and characteristic of the
hard-soft acid-base [19-20]. Chelating agents with N and
S groups are highly efficient for the selective sorption of
Au(III). The investigator reported that the modified silica
may improve the capacity and selectivity for Au(III) [19].
Amino acids are organic compounds containing
carboxylic (-COOH) and amine (-NH2) groups that can
act as ligands. Study on utilizing of amino acids (e.g.
glycine, valine, leucine and serine) as active sites to
modify chitosan has performed by Oshita et al. for
adsorption cationic and anionic species [21]. Glycine [22]
and lysine [20] as active sites to modify chitosan was
used for adsorption Au(III), Pt(IV) and Pd(II). The aim of
the present work is modification of silica with L-arginine
through sol-gel process by using a linker of GPTMS, and
the precursor of TEOS. L-arginine was chosen as the
modifier because it contains four amine groups per
molecule available as chelating agents. Amine groups
on L-arginine causes a high efficiency for the selectivity
of adsorption of Au(III). The synthesized material was
then used as an adsorbent for Au(III) ion in aqueous
solution. The adsorption kinetics of Au(III), the
adsorption isotherm and the selectivity toward Au(III) are
evaluated, as well.
EXPERIMENTAL SECTION
Materials
Materials used for preparation of adsorbent were
TEOS (Merck), L-arginine (Merck), GPTMS (Merck),
and ethanol 99.5% (v/v) (Univar). A solution of Au(III)
500 ppm was prepared in our laboratory by dissolving
gold in an aquaregia solution. Sodium hydroxide
(NaOH), hydrochloric acid (HCl), copper chloride
dihydrate (CuCl3∙2H2O), ferric chloride hexahydrate
(FeCl3∙6H2O)
and
zinc
nitrate
tetrahydrate
(Zn(NO3)2∙4H2O) in analytical reagent grade were
purchased from Merck, Germany without prior
treatment.
Instrumentation
The functional group of materials was identified
with FTIR spectrophotometer (Shimadzu IR prestige
21). Thermogravimetric method was used to calculate
the thermal weight loss of adsorbent at the temperature
range of 27–800 °C with heating rate of 10 °C/min (stapt-1600 TG-DSC/DTA linseis thermal analyzer). The
determination of metal ion concentrations was
performed
with
a
flame
atomic
absorption
spectrophotometer (FAAS, Shimadzu AA 6650). The
content of elements was determined with a Yanaco
CHN CORDER MT-6 Elemental Analyzer.
Procedure
Synthesis of L-arginine modified silica (SiO2-Arg)
Synthesis of silica modified with L-arginine was
performed by mixing 2 mL TEOS 4.5 M (9 mmol), 1 mL
ethanol, 1 mL GPTMS 4.5 M (4.5 mmol) and 0.9 M Larginine (1.568 g in 10 mL solution) at variation of
volume (1, 2, 3 and 4 mL). The mixture was stirred with
a magnetic stirrer for 2 h, and then allowed for 24 h.
The product was washed with 20 mL of distilled water,
and heated to a temperature of 60 °C for 6 h.
Adsorption
Adsorption was carried out in a batch system by
adding 10 mg of adsorbent in 10 mL solution of Au(III)
-1
15 mg L by varying contact time (from 5 to 120 min) at
pH 3.0. The mixture was stirred for 2 h and filtered.
Au(III) in supernatant was analyzed by FAAS and the
percentage of Au(III) adsorbed was calculated using
Eq. 1.
C Ce
P 100 x 0
C0
Sri Hastuti et al.
109
(1)
110
Indones. J. Chem., 2015, 15 (2), 108 - 115
Fig 1. The relation between percentage of absorbed
Au(III) and mol of L-arginine in synthesis of SiO2-Arg at
constant volume of TEOS (9 mmol) and GPTMS (4.5
mmol). Adsorption condition: 10 mL Au(III)15 ppm and
10 mg of adsorbent, pH 3.0. The inset shows the relation
between mol of L-arginine and pH filtrate
where P represents the amount of the metal ion
adsorbed (%); C0 and Ce are the initial and the final
–1
concentration of the metal ions (mg L ), respectively.
Several models of the kinetics (first order, second
order, pseudo-first order, and pseudo-second order)
were examined and the rate constants were calculated
to study the kinetics of the adsorption. Additionally,
adsorption in various concentrations of Au(III), in a range
–1
of 5–40 mg L at constant pH and contact time was
conducted, as well. The data was evaluated using
Langmuir [23] and Freundlich [24] equations to calculate
the adsorption capacity. The selectivity of the adsorbent
in adsorbing Au(III) ion was evaluated in the presence of
Cu(II), Fe(III) and Zn(II) ions at various pHs.
RESULT AND DISCUSSION
Effect of the Arginine Addition in SiO2-Arg Synthesis
on Au(III) Absorption
Active sites of L-arginine would affect the ability of
the adsorbent. The more number of L-arginine, the
greater adsorbent ability (SiO2-Arg) to absorb Au(III).
Fig. 1 shows that the absence of L-arginine on the silica,
the amount of Au(III) adsorbed metal ions is very low,
while the addition of L-arginine, the amount of absorbed
Au(III) is significantly increased. At the addition of Larginine more than 1 mL (0.9 mmol), the amount of the
absorbed Au(III) tends to be constant (76%), indicating
the maximum amount of L-arginine that can be bonded
on the silica through the linker. Excess of L-arginine
added in the sol-gel process dissolves during the
leaching, resulting in the increase of the solution pH. The
Sri Hastuti et al.
Fig 2. FTIR analysis spectra of SiO2, GPTMS, SiO2-Arg
Table 1. The composition of elements in SiO2 and
SiO2-Arg
Type
SiO2
SiO2-Arg
Weight (mg)
2.819
2.617
C (%)
0.38
22.35
H (%)
1.01
4.80
N (%)
0
2.81
N/C
0
0.13
inset in Fig. 1 shows the more L-arginine is added, the
higher the pH of the solution (mol L-arginine inset).
Therefore, the composition to prepare SiO2-Arg is
optimum at the mole ratio of GTPMS to Arg of 5:1 and
this proportion was used at the further experiment.
Characteristic of SiO2-Arg
Elemental analysis
Elemental analysis was conducted by a Yanaco
CHN CORDER MT-6 Elemental Analyzer to determine
the concentration of elements exist in the material. The
data of elemental analysis of SiO2 and SiO2-Arg are
presented in Table 1.
Table 1 shows that the SiO2-Arg containing 2.81%
nitrogen, whereas silica (SiO2) does not contain
nitrogen. The presence of nitrogen suggests that
arginine has been bound by silica through GPTMS
linker. The structure of SiO2-Arg can be predicted by
comparing the mass ratio of nitrogen to carbon (N/C) of
the theoretical and analysis. Ratio of the elemental
composition N/C analysis is closer to the theoretical
yield of 0.129. This condition suggests that the arginine
molecule binds five molecules of GPTMS. This is also
supported by the data of influences the amount of
arginine for adsorption of Au(III) that the mole ratio of
GTPMS:Arg is 5:1. The proposed structure of
interaction between GPTMS and arginine on silica can
be seen Fig. 3.
Functional group of SiO2-Arg
The presence of the functional group in (GPTMS),
and L-arginine-modified silica (SiO2-Arg) are shown in
111
Indones. J. Chem., 2015, 15 (2), 108 - 115
OH OH
Si
OH
O
OCH 3
5
H3 CO
Si
OCH3
H 2O
HO
+
O
OH
OH Si
OH
O
OH
H2 N
Si
HN
OH
OH Si
OH
NH
H2N
(Arg)
N
OH
HN
OH
O
OH
O
OH Si
OH
C2H5O
Si
OH
OH
OC 2H 5
+ -OH
H 2O
OH
C 2H 5O
(I)
+ 15 C2H5 OH
N
O
OH
O
(GPTMS)
O
OH
O
OH
O
NH
OH
C2H 5O
Si
OH + 4 C2H 5OH + 4 OH
(II)
OH
O
O Si
O
(I)
+
O
(II)
Polymerisation
Further Condensaion
Aging
O
O Si
O
O
O
O Si
O
O
OH
O
NH
OH
O
N
OH
N
HN
O
O
O Si
O
OH
(III)
OH
O
Si
O O O
Fig 3. Proposed reaction mechanism of L-arginine-modified silica synthesis
Fig. 2. Silica is characterized by the presence of two
-1
peaks at 3417 and 1091 cm from Si-OH and Si-O-Si
stretching vibration, respectively [25-26]. Similar to silica,
FTIR spectra of GPTMS also shows two peaks from
Si-OH and Si-O-Si stretching vibration at 3446 and
-1
-1
1105 cm , respectively. The peak at 2939 cm is the
aliphatic C-H stretching vibration and the peak at
-1
999 cm is the C-H stretching vibration of epoxy ring.
Furthermore the other evidence of epoxide groups was
indicated by peaks of C-C asymmetric and symmetric
-1
ring stretching vibration
at 908 and 1255 cm ,
respectively [25]. The formation of SiO2-Arg was
evidenced by the appearance of two new peaks at 1678
-1
and 1579 cm from asymmetric NH2 out-of-plane
bending vibration and C=O stretching vibration,
respectively [27]. Moreover the disappearance of the
peaks of the epoxide ring C-C asymmetric and
-1
symmetric stretching vibration at 908 and 1255 cm ,
respectively, confirms that the L-arginine molecule is
Sri Hastuti et al.
successfully attached on the silica surface by covalent
bond.
The formation model of L-arginine-modified silica
(SiO2-Arg) is described as a multi-step of hydrolysis
and condensation reactions (Fig. 3). The first stage is
an attack on the epoxide group by the amine followed
by hydrolysis illustrated in Fig. 3 reaction I. At the same
time TEOS also undergoes hydrolysis to form orthosilicic acid (Si(OH)4) in Fig. 3 reaction II. The second
stage is the condensation reaction of GPTMS-Arg and
(Si(OH)4) as shown in Fig. 3 reaction III.
Thermogravimetry Analysis (TGA)
Thermogravimetry
was
conducted
at
temperatures between 27–800 °C with the heating rate
of 10 °C/min. The TGA curve of SiO2 (Fig. 4) presented
a mass loss of 9.3100% (1.1720 mg) up to 100 °C,
related to adsorbed water, and presented a mass loss
of 2.7449% (0.3452 mg) above 200 °C, assigned to lost
112
Indones. J. Chem., 2015, 15 (2), 108 - 115
Fig 4. Thermogravimetric analysis curve of SiO2 and
SiO2-Arg
Fig 5. Effect of contact time on adsorption Au(III) onto
-1
SiO2-Arg (initial concentration: 15 mg L , mass dosage
0.01 g, pH 3.0)
Table 2. Constant rate of Au(III) adsorption on SiO2-Arg and linear coefficient in various kinetics models
Kinetic model
First order
Second order
Pseudo-first order
Pseudo-second order
Constant rate
–1
0.0070 min
–1
–1
0.2592 mM min
–1
0.0329 min
–1
–1
53197 g mol min
water from condensation of silanol groups to form
siloxane groups. The TGA curve of the SiO2-Arg (Fig. 4)
presents a mass loss of 6.2752% (0.7967 mg), assigned
to the adsorbed water up to 100 °C. At a temperature of
o
about 190-240 C, the weight of SiO2-Arg reduced
5.3767% (0.6826 mg) indicates the formation of siloxane
by releasing water molecules [27]. The weight of
L-arginine-modified silica (SiO2-Arg) reduced 35.4639%
(4.5027 mg) showing the decomposition by releasing
L-arginine molecules.
Adsorption of Au(III)
Kinetics of adsorption
Fig. 5 shows that the adsorption increases with
increasing of contact time and reached equilibrium within
60 min. Furthermore, the data of contact time variation
will be used to determine the kinetics of adsorption.
Adsorption kinetics describing the solute uptake rate is
one of the important characteristics that determine
absorption efficiency. First order (Eq. 2) [28], second
order (Eq. 3), pseudo-first order (Eq. 4) [29], and
pseudo-second order (Eq. 5) [30] were applied to
evaluate the experimental data. Kinetic model and
parameters for Au(III) adsorption are presented in Table
2.
ln Ce k1t ln C0
(2)
1
1
k 2t
Ce
C0
log qe qt logqe
Sri Hastuti et al.
(3)
k3
t
2.303
(4)
1
t
t
2
qt k 4qe qe
2
R
0.8489
0.8653
0.9012
0.9922
(5)
where Ce is concentration of Au(III) at equilibrium (mM),
C0 is initial concentration of Au(III) (mM); qe and qt are
-1
the amounts of Au(III) adsorbed (mol g ) at equilibrium
-1
and at any time t (mol g ), respectively; k1, k2, k3, and
-1
k4 are adsorption rate constant of first order (min ),
–1
–1
–1
second order (mM min ), pseudo-first order (min ),
–1
–1
and pseudo-second order (g mol min ), respectively.
As shown in Table 2, the correlation coefficient of
pseudo-second order higher than that of pseudo-firstorder. In addition, the value of qe calculated from the
-6
pseudo-second order as 5.63 x 10 mol/g is closer to
-6
the experimental qe value as 5.83 x 10 mol/g. In the
case of pseudo-first-order model, qe value calculated
-6
as 2.26 x 10 mol/g differs significantly from the
experimental qe value [20]. The qe was calculated by
plotting the log(qe - qt) vs. t for the pseudo-first-order
(Eq. 4). The slope of the linear plot t/qt vs. t yielded the
value of qe for the pseudo-second order (Eq. 5). The
experimental qe value is qt at time (60 min). Based on
the higher correlation coefficients and the agreement of
calculated qe value with experimental value, the
adsorption of Au(III) by SiO2-Arg tends to follow the
pseudo-second-order. This shows that the kinetic
model is chemical adsorption and does not involve
mass transfer in solution. It is more likely to predict that
the adsorption involves probable the use of shared
electrons between the noble metal cations and
adsorbent [20,22,31-32]
113
Indones. J. Chem., 2015, 15 (2), 108 - 115
Table 3. Langmuir and Freundlich parameters for Au(III) adsorption
Langmuir Parameters
Qm
ΔE
52.79 mg/g
24.197 kJ/mol
2
R
0.7853
2
R
0.9556
Freundlich Parameters
kf
0.022678
N
1.88
Table 4. Adsorption capacity of Au(III) on various reported adsorbents
No
1
2
3
4
5
Adsorben
Ionic imprinted amino-silica hybrid prepared from rice hull ash
L-lysine modified cross-linked chitosan resin
Glycine modified crosslinked chitosan resin
4-amino-4-nitro azobenzene modified chitosan
L-arginine-modified silica (SiO2-Arg)
Fig 6. Effect of pH on the adsorption selectivity Au(III)
onto SiO2-Arg with the presence of Cu(II), Zn(II) and
Fe(III) by varying the pH (mass dosage 0.01 g, 1 h)
Adsorption isotherm
Adsorption was conducted at various initial
concentrations of Au(III) 10 mL, ranging from 5 to 40
mg/L with adsorbent 10 mg at pH 3.0. Experimental data
was evaluated using Langmuir and Freundlich isotherm
models. Langmuir isotherm is based on monolayer
adsorption on the active sites of the adsorbent. On the
other hand, Freundlich isotherm describes the
adsorption on heterogeneous (multiple-layer) surface
with uniform energy. The relationship between the
Langmuir and Freundlich parameters with regression
coefficients was shown in Table 3. Correlation coefficient
values indicate that the Freundlich model is more
suitable than the Langmuir model. The adsorption
capacity data of SiO2-Arg compared to other adsorbents
reported can be seen in Table 4.
Table 4 compares the adsorption capacity of
various types of adsorbents for the adsorption of Au(III).
The difference in the absorption of ions Au(III) at various
adsorbent were caused by characteristics (functional
groups, surface area, particle size, etc.) of the
adsorbent.
Sri Hastuti et al.
Qm (mg/g)
76.14
70.34
169.98
69.93
52.79
Reference
[19]
[20]
[22]
[31]
This work
Selectivity of adsorption
Effect of pH on the adsorption selectivity Au(III) by
SiO2-Arg was studied together with the presence of
Cu(II), Fe(III) and Zn(II) by varying the pHs. The results
are presented in Fig. 6.
The mechanism of adsorption of Au(III) could be
through electrostatic force, ion exchange and chelation.
The nature of the mechanism depends on several
parameters such as pH, metal ions and adsorbent
properties. Au(III) reacts with hydrochloric acid to form
complex anion, [AuCl4] [20,22,31-32]. The amine group
can contribute to form a chelate with metal ions, but
protonation that occurs will significantly reduces its
ability to form a chelate. Thus most of the absorption of
the metal Au(III) is due to the electrostatic attraction of
metal complex anions by protonated amine groups.
The mechanism of adsorption in acid solution to Au(III)
on the SiO2-Arg was assumed to occur due to
electrostatic force and ion exchange.
The interaction between the metal ions and
protonated sites of SiO2-Arg is shown in Eq.(6-8)
(6)
R - NH2 + HCl RNH3 + ClAu(III) + 4HCl AuCl4 + 4H+
-
(7)
-
RNH3 + AuCl4 + Cl
(8)
At pH > 5, the decrease of Au(III) adsorption is
likely to occur due to the electrostatic repulsion
between the surface sites of the adsorbent and metal
ions. In addition, this might occur due to the reduction
of anion species Au(III) chloride caused by OH
replacement, subsequently form a precipitate of Au(III)
hydroxide as shown in Eq. 9 [19]:
AuCl4 + 4OH- Au(OH)4 + 4Cl- Au(OH)3(S) + OH- + 4Cl- (9)
RNH3 Cl + AuCl4
At lower pH, the decrease adsorption of Au(III)
was caused by reducing of [AuCl4] species more likely
forming HAuCl4 as shown in Eq.10. Several studies in
literatures reported that a highest adsorption of Au(III)
on amino based materials was obtained at pH of 2.04.0 [20,22, 31].
AuCl4 + HCl- HAuCl4 + Cl(10)
114
Indones. J. Chem., 2015, 15 (2), 108 - 115
The optimum adsorption selectivity is achieved at
pH 3.0. At optimum pH 3, amine groups (–NH2) in the
surface SiO2-Arg are protonated to form ammonium
+
groups (–NH3 ). In that condition, Au(III) ions form
–
complexes of [AuCl4] anion. The adsorption of Au(III) is
due to the electrostatic attraction of metal complex
anions by protonated amine groups. On the contrary,
Cu(II) and Zn(II) ions may not be adsorbed because the
electrostatic repulsion between the surface sites of the
adsorbent and metal ions. Thus Au(III) may be adsorbed
selectively from solution containing a mixture of Au(III),
Cu(II) and Zn(II) on SiO2-Arg. Whereas Au(III) is less
selective towards Fe(III) compared with Cu(II) and Zn(II).
At pH 3.0, Fe(III) tend to interact with OH ions to form
2+
hydroxo complex ions [FeOH] compared with Zn(II)
and Cu(II) ions. This is due to the solubility constants
(Ksp) of Fe(OH)3 is smaller than Zn(OH)2 and Cu(OH)2.
-17
-19
Ksp of Zn(OH)2, Cu(OH)2 and Fe(OH)3 are 5.10 , 2.10
-37
2+
and 1.10 , respectively [33]. The amount of [FeOH] in
2+
solution can form hydrogen bonds between [FeOH]
with the OH group of silanol of the adsorbent. Thus
2+
[FeOH] ions are more strongly bound to the SiO2-Arg
compared Cu(II) and Zn(II) ions. The adsorption
selectivity for (Au-Zn) and (Au-Cu) were higher than
those of (Au-Fe), showing the following order: (Au-Zn) =
(Au-Cu) > (Au-Fe) (in Fig. 6).
CONCLUSION
L-arginine-modified silica has been simply
prepared through sol gel process with the linker 3glycidoxypropyl-trimethoxysilane
using
tetraethyl
orthosilicate as the precursor and L-arginine solution.
The adsorption capacity obtained at optimum pH (3.0) is
equal to 52.79 mg/g. The kinetic study indicates that the
pseudo-second-order model provides better correlation
than the pseudo-first-order one; this suggests that the
rate-limiting step may be chemical sorption. SiO2-Arg
was selective for Au(III) adsorption toward Cu(II), and
Zn(II) with the selectivity order of (Au-Zn) = (Au-Cu) >
(Au-Fe). SiO2-Arg material provides a prospective
application as an effective adsorbent for metal ions as
well as gold(III).
ACKNOWLEDGEMENT
The authors are grateful to the Directorate of
Higher Education, Ministry of National Education,
Republic of Indonesia through Doctoral Scholarships.
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Indones. J. Chem., 2015, 15 (2), 108 - 115
L-ARGININE-MODIFIED SILICA FOR ADSORPTION OF GOLD(III)
Sri Hastuti1,*, Nuryono2, and Agus Kuncaka2
1
Department of Chemistry, Faculty of Mathematics and Natural Sciences, Sebelas Maret University,
Jl. Ir. Sutami 36 A Kentingan, Surakarta 57126, Indonesia
2
Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada,
Sekip Utara, Yogyakarta 55281, Indonesia
Received December 1, 2014; Accepted March 17, 2015
ABSTRACT
In this research, L-arginine-modified silica (SiO2-Arg) with 3-glycidoxypropyl-trimethoxysilane (GPTMS) as the
linking agent has been synthesized through sol gel process for adsorption of Au(III) in aqueous solution. Tetraethyl
orthosilicate (TEOS) as the silica source precursor, L-arginine solution 0.9 M with various volume ratios and the
linking agent were mixed together to form a gel. SiO2-Arg was characterized using Fourier transform infrared (FTIR)
spectrophotometer, thermogravimetric analysis (TGA), and an elemental analysis. Adsorption was carried out in a
batch system under various experimental conditions including contact time and initial concentration of metal Au(III).
The selectivity of adsorbent toward Au(III) was examined in the presence of Cu(II), Fe(III), and Zn(II) ion at various
pHs. Results of characterization showed that silica has been successfully modified with L-arginine. The optimum
adsorption of Au(III) on SiO2-Arg was obtained at pH of 3.0 and the adsorption isotherm of Au(III) on SiO2-Arg gives
the adsorption capacity of 52.79 mg/g. The kinetic study demonstrates that the adsorption of Au(III) ion follows
–1
–1
pseudo-second order with the rate constant of 53197 g mol min . The selectivity order of Au-Zn = Au-Cu > Au-Fe.
This sol-gel preparation is simple and provides prospective application of SiO2-Arg material as an effective
adsorbent for metal ions particularly gold(III).
Keywords: silica; modification; L-arginine; adsorption; gold(III)
ABSTRAK
Dalam penelitian ini, silika termodifikasi L-arginin (SiO2-Arg) dengan 3-glisidoksipropil-trimetoksisilan (GPTMS)
sebagai penggandeng telah disintesis melalui proses sol gel untuk adsorpsi Au(III) dalam larutan air. Tetraetil
ortosilikat (TEOS) sebagai prekursor sumber silika, larutan L-arginin 0,9 M dengan berbagai perbandingan volume
dan zat penggandeng dicampur bersama-sama untuk membentuk gel. SiO2-Arg dikarakterisasi menggunakan
spektropotometer Infra Merah, analisis termogravimetri, dan analisis unsur. Adsorpsi dilakukan dalam sistem batch
pada berbagai kondisi termasuk waktu kontak dan konsentrasi awal ion logam Au(III). Selektivitas adsorben
terhadap Au(III) diuji dengan hadirnya ion logam Cu(II), Fe(III), dan Zn(II) pada berbagai pH. Hasil karakterisasi
menunjukkan bahwa silika telah berhasil dimodifikasi dengan L-arginin. Adsorpsi optimum Au(III) pada SiO2-Arg
diperoleh pada pH 3,0 dan isoterm adsorpsi Au(III) pada SiO2-Arg memberikan kapasitas adsorpsi 52,79 mg/g. Studi
–1
kinetika menunjukkan bahwa adsorpsi ion Au(III) mengikuti pseudo orde dua dengan konstanta laju 53197 g mol
–1
min . Urutan selektivitas Au-Zn = Au-Cu > Au-Fe. Preparasi sol-gel ini sederhana dan memberikan aplikasi yang
prospektif dari material SiO2-Arg sebagai adsorben yang efektif terutama untuk ion logam emas(III).
Kata Kunci: silica; modifikasi; L-arginin; adsorpsi; emas(III)
INTRODUCTION
Recovery of gold from waste water is more
interesting than that of most other metals [1]. Moreover
gold recovery from aqueous solutions has received
significant attention because gold is present in
appreciable amounts in electronic parts and plating
materials [2]. The content of gold in electronic waste is
greater than the gold content of the ore. For example,
gold concentrations in mobile phone handsets and
* Corresponding author. Tel/Fax : +62-85725590941
Email address : hastuti.uns@gmail.com
Sri Hastuti et al.
computer circuit boards are about 300-350 and
200-250 g/t, respectively [3] while the ore contains gold
in a range of 5-30 g/t [4].
There are several methods for the removal of
gold(III) from aqueous solutions, such as solvent
extraction [5-6] and adsorption [1]. The last method
seems to be the most suitable for the recovery of
gold(III) in the case of low concentration due to low
cost and high efficiency. In development of adsorbent,
silica gel is very important material because it can be
Indones. J. Chem., 2015, 15 (2), 108 - 115
used as a support material possesses some advantages
such as high mechanical and chemical properties, high
porosity, large surface, no swelling, resistant in microbial
attack, resistant in decay radiation and heat-stable
[7-10]. In addition, chelating agents can be easily
bonded chemically to the support, that affording a higher
stability [7]. Chemically, silica contains functional groups
of silanol (-Si-OH) and siloxane (-Si-O-Si-) on the
surface, allowing to be modified with active groups
specific to certain targets [11-13].
One way to improve the ability of silica as an
adsorbent is by surface modification with organic
functional groups. Modification may be chemically
conducted by using organosilane compound as a linker,
which then is followed by attachment of the active group
to the linker. The various linker has been used for the
modification of silica, such as 3-aminopropyltriethoxy
silane (APTES) [14], 3-chloropropyltrimethoxysilane
(CPTMS) [15], 3-aminopropyltrimethoxysilane (APTMS)
[16-17], 3-glycidoxypropyltrimethoxysilane (GPTMS)
[18]. Donor atoms commonly used as the active group
include nitrogen (e.g. amine, azo, amide and nitrile),
oxygen (e.g carboxyl, hydroxyl, carbonyl and
phosphoryl), and sulfur (e.g. thiocarbamate, thioethers,
and mercapto) [8]. The selectivity of the immobilized
surface towards Au(III) depends on various factors such
as kind of complexing agent and characteristic of the
hard-soft acid-base [19-20]. Chelating agents with N and
S groups are highly efficient for the selective sorption of
Au(III). The investigator reported that the modified silica
may improve the capacity and selectivity for Au(III) [19].
Amino acids are organic compounds containing
carboxylic (-COOH) and amine (-NH2) groups that can
act as ligands. Study on utilizing of amino acids (e.g.
glycine, valine, leucine and serine) as active sites to
modify chitosan has performed by Oshita et al. for
adsorption cationic and anionic species [21]. Glycine [22]
and lysine [20] as active sites to modify chitosan was
used for adsorption Au(III), Pt(IV) and Pd(II). The aim of
the present work is modification of silica with L-arginine
through sol-gel process by using a linker of GPTMS, and
the precursor of TEOS. L-arginine was chosen as the
modifier because it contains four amine groups per
molecule available as chelating agents. Amine groups
on L-arginine causes a high efficiency for the selectivity
of adsorption of Au(III). The synthesized material was
then used as an adsorbent for Au(III) ion in aqueous
solution. The adsorption kinetics of Au(III), the
adsorption isotherm and the selectivity toward Au(III) are
evaluated, as well.
EXPERIMENTAL SECTION
Materials
Materials used for preparation of adsorbent were
TEOS (Merck), L-arginine (Merck), GPTMS (Merck),
and ethanol 99.5% (v/v) (Univar). A solution of Au(III)
500 ppm was prepared in our laboratory by dissolving
gold in an aquaregia solution. Sodium hydroxide
(NaOH), hydrochloric acid (HCl), copper chloride
dihydrate (CuCl3∙2H2O), ferric chloride hexahydrate
(FeCl3∙6H2O)
and
zinc
nitrate
tetrahydrate
(Zn(NO3)2∙4H2O) in analytical reagent grade were
purchased from Merck, Germany without prior
treatment.
Instrumentation
The functional group of materials was identified
with FTIR spectrophotometer (Shimadzu IR prestige
21). Thermogravimetric method was used to calculate
the thermal weight loss of adsorbent at the temperature
range of 27–800 °C with heating rate of 10 °C/min (stapt-1600 TG-DSC/DTA linseis thermal analyzer). The
determination of metal ion concentrations was
performed
with
a
flame
atomic
absorption
spectrophotometer (FAAS, Shimadzu AA 6650). The
content of elements was determined with a Yanaco
CHN CORDER MT-6 Elemental Analyzer.
Procedure
Synthesis of L-arginine modified silica (SiO2-Arg)
Synthesis of silica modified with L-arginine was
performed by mixing 2 mL TEOS 4.5 M (9 mmol), 1 mL
ethanol, 1 mL GPTMS 4.5 M (4.5 mmol) and 0.9 M Larginine (1.568 g in 10 mL solution) at variation of
volume (1, 2, 3 and 4 mL). The mixture was stirred with
a magnetic stirrer for 2 h, and then allowed for 24 h.
The product was washed with 20 mL of distilled water,
and heated to a temperature of 60 °C for 6 h.
Adsorption
Adsorption was carried out in a batch system by
adding 10 mg of adsorbent in 10 mL solution of Au(III)
-1
15 mg L by varying contact time (from 5 to 120 min) at
pH 3.0. The mixture was stirred for 2 h and filtered.
Au(III) in supernatant was analyzed by FAAS and the
percentage of Au(III) adsorbed was calculated using
Eq. 1.
C Ce
P 100 x 0
C0
Sri Hastuti et al.
109
(1)
110
Indones. J. Chem., 2015, 15 (2), 108 - 115
Fig 1. The relation between percentage of absorbed
Au(III) and mol of L-arginine in synthesis of SiO2-Arg at
constant volume of TEOS (9 mmol) and GPTMS (4.5
mmol). Adsorption condition: 10 mL Au(III)15 ppm and
10 mg of adsorbent, pH 3.0. The inset shows the relation
between mol of L-arginine and pH filtrate
where P represents the amount of the metal ion
adsorbed (%); C0 and Ce are the initial and the final
–1
concentration of the metal ions (mg L ), respectively.
Several models of the kinetics (first order, second
order, pseudo-first order, and pseudo-second order)
were examined and the rate constants were calculated
to study the kinetics of the adsorption. Additionally,
adsorption in various concentrations of Au(III), in a range
–1
of 5–40 mg L at constant pH and contact time was
conducted, as well. The data was evaluated using
Langmuir [23] and Freundlich [24] equations to calculate
the adsorption capacity. The selectivity of the adsorbent
in adsorbing Au(III) ion was evaluated in the presence of
Cu(II), Fe(III) and Zn(II) ions at various pHs.
RESULT AND DISCUSSION
Effect of the Arginine Addition in SiO2-Arg Synthesis
on Au(III) Absorption
Active sites of L-arginine would affect the ability of
the adsorbent. The more number of L-arginine, the
greater adsorbent ability (SiO2-Arg) to absorb Au(III).
Fig. 1 shows that the absence of L-arginine on the silica,
the amount of Au(III) adsorbed metal ions is very low,
while the addition of L-arginine, the amount of absorbed
Au(III) is significantly increased. At the addition of Larginine more than 1 mL (0.9 mmol), the amount of the
absorbed Au(III) tends to be constant (76%), indicating
the maximum amount of L-arginine that can be bonded
on the silica through the linker. Excess of L-arginine
added in the sol-gel process dissolves during the
leaching, resulting in the increase of the solution pH. The
Sri Hastuti et al.
Fig 2. FTIR analysis spectra of SiO2, GPTMS, SiO2-Arg
Table 1. The composition of elements in SiO2 and
SiO2-Arg
Type
SiO2
SiO2-Arg
Weight (mg)
2.819
2.617
C (%)
0.38
22.35
H (%)
1.01
4.80
N (%)
0
2.81
N/C
0
0.13
inset in Fig. 1 shows the more L-arginine is added, the
higher the pH of the solution (mol L-arginine inset).
Therefore, the composition to prepare SiO2-Arg is
optimum at the mole ratio of GTPMS to Arg of 5:1 and
this proportion was used at the further experiment.
Characteristic of SiO2-Arg
Elemental analysis
Elemental analysis was conducted by a Yanaco
CHN CORDER MT-6 Elemental Analyzer to determine
the concentration of elements exist in the material. The
data of elemental analysis of SiO2 and SiO2-Arg are
presented in Table 1.
Table 1 shows that the SiO2-Arg containing 2.81%
nitrogen, whereas silica (SiO2) does not contain
nitrogen. The presence of nitrogen suggests that
arginine has been bound by silica through GPTMS
linker. The structure of SiO2-Arg can be predicted by
comparing the mass ratio of nitrogen to carbon (N/C) of
the theoretical and analysis. Ratio of the elemental
composition N/C analysis is closer to the theoretical
yield of 0.129. This condition suggests that the arginine
molecule binds five molecules of GPTMS. This is also
supported by the data of influences the amount of
arginine for adsorption of Au(III) that the mole ratio of
GTPMS:Arg is 5:1. The proposed structure of
interaction between GPTMS and arginine on silica can
be seen Fig. 3.
Functional group of SiO2-Arg
The presence of the functional group in (GPTMS),
and L-arginine-modified silica (SiO2-Arg) are shown in
111
Indones. J. Chem., 2015, 15 (2), 108 - 115
OH OH
Si
OH
O
OCH 3
5
H3 CO
Si
OCH3
H 2O
HO
+
O
OH
OH Si
OH
O
OH
H2 N
Si
HN
OH
OH Si
OH
NH
H2N
(Arg)
N
OH
HN
OH
O
OH
O
OH Si
OH
C2H5O
Si
OH
OH
OC 2H 5
+ -OH
H 2O
OH
C 2H 5O
(I)
+ 15 C2H5 OH
N
O
OH
O
(GPTMS)
O
OH
O
OH
O
NH
OH
C2H 5O
Si
OH + 4 C2H 5OH + 4 OH
(II)
OH
O
O Si
O
(I)
+
O
(II)
Polymerisation
Further Condensaion
Aging
O
O Si
O
O
O
O Si
O
O
OH
O
NH
OH
O
N
OH
N
HN
O
O
O Si
O
OH
(III)
OH
O
Si
O O O
Fig 3. Proposed reaction mechanism of L-arginine-modified silica synthesis
Fig. 2. Silica is characterized by the presence of two
-1
peaks at 3417 and 1091 cm from Si-OH and Si-O-Si
stretching vibration, respectively [25-26]. Similar to silica,
FTIR spectra of GPTMS also shows two peaks from
Si-OH and Si-O-Si stretching vibration at 3446 and
-1
-1
1105 cm , respectively. The peak at 2939 cm is the
aliphatic C-H stretching vibration and the peak at
-1
999 cm is the C-H stretching vibration of epoxy ring.
Furthermore the other evidence of epoxide groups was
indicated by peaks of C-C asymmetric and symmetric
-1
ring stretching vibration
at 908 and 1255 cm ,
respectively [25]. The formation of SiO2-Arg was
evidenced by the appearance of two new peaks at 1678
-1
and 1579 cm from asymmetric NH2 out-of-plane
bending vibration and C=O stretching vibration,
respectively [27]. Moreover the disappearance of the
peaks of the epoxide ring C-C asymmetric and
-1
symmetric stretching vibration at 908 and 1255 cm ,
respectively, confirms that the L-arginine molecule is
Sri Hastuti et al.
successfully attached on the silica surface by covalent
bond.
The formation model of L-arginine-modified silica
(SiO2-Arg) is described as a multi-step of hydrolysis
and condensation reactions (Fig. 3). The first stage is
an attack on the epoxide group by the amine followed
by hydrolysis illustrated in Fig. 3 reaction I. At the same
time TEOS also undergoes hydrolysis to form orthosilicic acid (Si(OH)4) in Fig. 3 reaction II. The second
stage is the condensation reaction of GPTMS-Arg and
(Si(OH)4) as shown in Fig. 3 reaction III.
Thermogravimetry Analysis (TGA)
Thermogravimetry
was
conducted
at
temperatures between 27–800 °C with the heating rate
of 10 °C/min. The TGA curve of SiO2 (Fig. 4) presented
a mass loss of 9.3100% (1.1720 mg) up to 100 °C,
related to adsorbed water, and presented a mass loss
of 2.7449% (0.3452 mg) above 200 °C, assigned to lost
112
Indones. J. Chem., 2015, 15 (2), 108 - 115
Fig 4. Thermogravimetric analysis curve of SiO2 and
SiO2-Arg
Fig 5. Effect of contact time on adsorption Au(III) onto
-1
SiO2-Arg (initial concentration: 15 mg L , mass dosage
0.01 g, pH 3.0)
Table 2. Constant rate of Au(III) adsorption on SiO2-Arg and linear coefficient in various kinetics models
Kinetic model
First order
Second order
Pseudo-first order
Pseudo-second order
Constant rate
–1
0.0070 min
–1
–1
0.2592 mM min
–1
0.0329 min
–1
–1
53197 g mol min
water from condensation of silanol groups to form
siloxane groups. The TGA curve of the SiO2-Arg (Fig. 4)
presents a mass loss of 6.2752% (0.7967 mg), assigned
to the adsorbed water up to 100 °C. At a temperature of
o
about 190-240 C, the weight of SiO2-Arg reduced
5.3767% (0.6826 mg) indicates the formation of siloxane
by releasing water molecules [27]. The weight of
L-arginine-modified silica (SiO2-Arg) reduced 35.4639%
(4.5027 mg) showing the decomposition by releasing
L-arginine molecules.
Adsorption of Au(III)
Kinetics of adsorption
Fig. 5 shows that the adsorption increases with
increasing of contact time and reached equilibrium within
60 min. Furthermore, the data of contact time variation
will be used to determine the kinetics of adsorption.
Adsorption kinetics describing the solute uptake rate is
one of the important characteristics that determine
absorption efficiency. First order (Eq. 2) [28], second
order (Eq. 3), pseudo-first order (Eq. 4) [29], and
pseudo-second order (Eq. 5) [30] were applied to
evaluate the experimental data. Kinetic model and
parameters for Au(III) adsorption are presented in Table
2.
ln Ce k1t ln C0
(2)
1
1
k 2t
Ce
C0
log qe qt logqe
Sri Hastuti et al.
(3)
k3
t
2.303
(4)
1
t
t
2
qt k 4qe qe
2
R
0.8489
0.8653
0.9012
0.9922
(5)
where Ce is concentration of Au(III) at equilibrium (mM),
C0 is initial concentration of Au(III) (mM); qe and qt are
-1
the amounts of Au(III) adsorbed (mol g ) at equilibrium
-1
and at any time t (mol g ), respectively; k1, k2, k3, and
-1
k4 are adsorption rate constant of first order (min ),
–1
–1
–1
second order (mM min ), pseudo-first order (min ),
–1
–1
and pseudo-second order (g mol min ), respectively.
As shown in Table 2, the correlation coefficient of
pseudo-second order higher than that of pseudo-firstorder. In addition, the value of qe calculated from the
-6
pseudo-second order as 5.63 x 10 mol/g is closer to
-6
the experimental qe value as 5.83 x 10 mol/g. In the
case of pseudo-first-order model, qe value calculated
-6
as 2.26 x 10 mol/g differs significantly from the
experimental qe value [20]. The qe was calculated by
plotting the log(qe - qt) vs. t for the pseudo-first-order
(Eq. 4). The slope of the linear plot t/qt vs. t yielded the
value of qe for the pseudo-second order (Eq. 5). The
experimental qe value is qt at time (60 min). Based on
the higher correlation coefficients and the agreement of
calculated qe value with experimental value, the
adsorption of Au(III) by SiO2-Arg tends to follow the
pseudo-second-order. This shows that the kinetic
model is chemical adsorption and does not involve
mass transfer in solution. It is more likely to predict that
the adsorption involves probable the use of shared
electrons between the noble metal cations and
adsorbent [20,22,31-32]
113
Indones. J. Chem., 2015, 15 (2), 108 - 115
Table 3. Langmuir and Freundlich parameters for Au(III) adsorption
Langmuir Parameters
Qm
ΔE
52.79 mg/g
24.197 kJ/mol
2
R
0.7853
2
R
0.9556
Freundlich Parameters
kf
0.022678
N
1.88
Table 4. Adsorption capacity of Au(III) on various reported adsorbents
No
1
2
3
4
5
Adsorben
Ionic imprinted amino-silica hybrid prepared from rice hull ash
L-lysine modified cross-linked chitosan resin
Glycine modified crosslinked chitosan resin
4-amino-4-nitro azobenzene modified chitosan
L-arginine-modified silica (SiO2-Arg)
Fig 6. Effect of pH on the adsorption selectivity Au(III)
onto SiO2-Arg with the presence of Cu(II), Zn(II) and
Fe(III) by varying the pH (mass dosage 0.01 g, 1 h)
Adsorption isotherm
Adsorption was conducted at various initial
concentrations of Au(III) 10 mL, ranging from 5 to 40
mg/L with adsorbent 10 mg at pH 3.0. Experimental data
was evaluated using Langmuir and Freundlich isotherm
models. Langmuir isotherm is based on monolayer
adsorption on the active sites of the adsorbent. On the
other hand, Freundlich isotherm describes the
adsorption on heterogeneous (multiple-layer) surface
with uniform energy. The relationship between the
Langmuir and Freundlich parameters with regression
coefficients was shown in Table 3. Correlation coefficient
values indicate that the Freundlich model is more
suitable than the Langmuir model. The adsorption
capacity data of SiO2-Arg compared to other adsorbents
reported can be seen in Table 4.
Table 4 compares the adsorption capacity of
various types of adsorbents for the adsorption of Au(III).
The difference in the absorption of ions Au(III) at various
adsorbent were caused by characteristics (functional
groups, surface area, particle size, etc.) of the
adsorbent.
Sri Hastuti et al.
Qm (mg/g)
76.14
70.34
169.98
69.93
52.79
Reference
[19]
[20]
[22]
[31]
This work
Selectivity of adsorption
Effect of pH on the adsorption selectivity Au(III) by
SiO2-Arg was studied together with the presence of
Cu(II), Fe(III) and Zn(II) by varying the pHs. The results
are presented in Fig. 6.
The mechanism of adsorption of Au(III) could be
through electrostatic force, ion exchange and chelation.
The nature of the mechanism depends on several
parameters such as pH, metal ions and adsorbent
properties. Au(III) reacts with hydrochloric acid to form
complex anion, [AuCl4] [20,22,31-32]. The amine group
can contribute to form a chelate with metal ions, but
protonation that occurs will significantly reduces its
ability to form a chelate. Thus most of the absorption of
the metal Au(III) is due to the electrostatic attraction of
metal complex anions by protonated amine groups.
The mechanism of adsorption in acid solution to Au(III)
on the SiO2-Arg was assumed to occur due to
electrostatic force and ion exchange.
The interaction between the metal ions and
protonated sites of SiO2-Arg is shown in Eq.(6-8)
(6)
R - NH2 + HCl RNH3 + ClAu(III) + 4HCl AuCl4 + 4H+
-
(7)
-
RNH3 + AuCl4 + Cl
(8)
At pH > 5, the decrease of Au(III) adsorption is
likely to occur due to the electrostatic repulsion
between the surface sites of the adsorbent and metal
ions. In addition, this might occur due to the reduction
of anion species Au(III) chloride caused by OH
replacement, subsequently form a precipitate of Au(III)
hydroxide as shown in Eq. 9 [19]:
AuCl4 + 4OH- Au(OH)4 + 4Cl- Au(OH)3(S) + OH- + 4Cl- (9)
RNH3 Cl + AuCl4
At lower pH, the decrease adsorption of Au(III)
was caused by reducing of [AuCl4] species more likely
forming HAuCl4 as shown in Eq.10. Several studies in
literatures reported that a highest adsorption of Au(III)
on amino based materials was obtained at pH of 2.04.0 [20,22, 31].
AuCl4 + HCl- HAuCl4 + Cl(10)
114
Indones. J. Chem., 2015, 15 (2), 108 - 115
The optimum adsorption selectivity is achieved at
pH 3.0. At optimum pH 3, amine groups (–NH2) in the
surface SiO2-Arg are protonated to form ammonium
+
groups (–NH3 ). In that condition, Au(III) ions form
–
complexes of [AuCl4] anion. The adsorption of Au(III) is
due to the electrostatic attraction of metal complex
anions by protonated amine groups. On the contrary,
Cu(II) and Zn(II) ions may not be adsorbed because the
electrostatic repulsion between the surface sites of the
adsorbent and metal ions. Thus Au(III) may be adsorbed
selectively from solution containing a mixture of Au(III),
Cu(II) and Zn(II) on SiO2-Arg. Whereas Au(III) is less
selective towards Fe(III) compared with Cu(II) and Zn(II).
At pH 3.0, Fe(III) tend to interact with OH ions to form
2+
hydroxo complex ions [FeOH] compared with Zn(II)
and Cu(II) ions. This is due to the solubility constants
(Ksp) of Fe(OH)3 is smaller than Zn(OH)2 and Cu(OH)2.
-17
-19
Ksp of Zn(OH)2, Cu(OH)2 and Fe(OH)3 are 5.10 , 2.10
-37
2+
and 1.10 , respectively [33]. The amount of [FeOH] in
2+
solution can form hydrogen bonds between [FeOH]
with the OH group of silanol of the adsorbent. Thus
2+
[FeOH] ions are more strongly bound to the SiO2-Arg
compared Cu(II) and Zn(II) ions. The adsorption
selectivity for (Au-Zn) and (Au-Cu) were higher than
those of (Au-Fe), showing the following order: (Au-Zn) =
(Au-Cu) > (Au-Fe) (in Fig. 6).
CONCLUSION
L-arginine-modified silica has been simply
prepared through sol gel process with the linker 3glycidoxypropyl-trimethoxysilane
using
tetraethyl
orthosilicate as the precursor and L-arginine solution.
The adsorption capacity obtained at optimum pH (3.0) is
equal to 52.79 mg/g. The kinetic study indicates that the
pseudo-second-order model provides better correlation
than the pseudo-first-order one; this suggests that the
rate-limiting step may be chemical sorption. SiO2-Arg
was selective for Au(III) adsorption toward Cu(II), and
Zn(II) with the selectivity order of (Au-Zn) = (Au-Cu) >
(Au-Fe). SiO2-Arg material provides a prospective
application as an effective adsorbent for metal ions as
well as gold(III).
ACKNOWLEDGEMENT
The authors are grateful to the Directorate of
Higher Education, Ministry of National Education,
Republic of Indonesia through Doctoral Scholarships.
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